Motors have been used for a wide variety of consumer and industrial applications such as transportation, manufacturing and home appliances. While motors have been in existence for many years, more sophisticated methods of motor control have been employed as motor technology has advanced. In particular, variable speed induction motors and permanent magnet brushless DC motors have become more prevalent. Such systems typically require solid-state driver devices, motor position sensors and microprocessor based motor controllers.
In a typical three-phase variable speed induction motor, time varying currents are supplied to three stator windings to create a rotating magnetic field. This time varying magnetic field induces the rotor containing a permanent magnet to turn. The speed of the motor is determined, in part, by the frequency of the time varying current controlled by a motor control unit.
In applications where a motor is exposed to a variable load, for example in an electric bicycle or a kitchen appliance, the current required by the stator windings are proportional to the physical load being driven. In cases where a motor is prevented from turning, for example, in an accident or under an extremely heavy load, or in cases where the motor experiences a short circuit, current drivers within the motor controller may experience a high degree of electrical stress and sink high currents. If the currents provided by the motor controller are too high, the motor controller can rapidly become damaged. High motor currents also lead to high power consumption, poor motor efficiency, frequent motor breakdowns.
Some prior art systems that have addressed the problem of motor high currents under high loads by limiting the current to the motor via a closed current control loop. A problem with relying on closed-loop control to limit motor current is that the closed-loop response time may be too slow to avoid damage to the motor and motor controller. Other prior art systems employ complicated current sensors that add cost and complexity to the motor control system.
In the field of motors, what is needed are fast and cost efficient systems and methods of detecting high current conditions within a motor.